$ D = \left[\begin{array}{rr}5 & 1 \\ -1 & 3 \\ 2 & -1\end{array}\right]$ $ v = \left[\begin{array}{r}1 \\ 4\end{array}\right]$ What is $ D v$ ?
Answer: Because $ D$ has dimensions $(3\times2)$ and $ v$ has dimensions $(2\times1)$ , the answer matrix will have dimensions $(3\times1)$ $ D v = \left[\begin{array}{rr}{5} & {1} \\ {-1} & {3} \\ \color{gray}{2} & \color{gray}{-1}\end{array}\right] \left[\begin{array}{r}{1} \\ {4}\end{array}\right] = \left[\begin{array}{r}? \\ ? \\ ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ D$ , with the corresponding elements in column $j$ of the second matrix, $ v$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ D$ with the first element in ${\text{column }1}$ of $ v$ , then multiply the second element in ${\text{row }1}$ of $ D$ with the second element in ${\text{column }1}$ of $ v$ , and so on. Add the products together. $ \left[\begin{array}{r}{5}\cdot{1}+{1}\cdot{4} \\ ? \\ ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ D$ with the corresponding elements in ${\text{column }1}$ of $ v$ and add the products together. $ \left[\begin{array}{r}{5}\cdot{1}+{1}\cdot{4} \\ {-1}\cdot{1}+{3}\cdot{4} \\ ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{r}{5}\cdot{1}+{1}\cdot{4} \\ {-1}\cdot{1}+{3}\cdot{4} \\ \color{gray}{2}\cdot{1}+\color{gray}{-1}\cdot{4}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{r}9 \\ 11 \\ -2\end{array}\right] $